2.4. Fe II emission and narrow line Seyfert 1 galaxies

2.4.1. Fe II emission

Nearly all broad line AGNs have optical Fe II emission in their spectrum ([322]). The blend of Fe II lines between H and H consists of lines in multiplets 26,37,38,43 and 44 ([166]). The Fe II strength is usually measured by the quantity R4570 = Fe II 4570 / H, i.e. the relative flux in the 4570 blend measured between 4434 and 4684 (see for instance [55]) and in H. Typical AGNs have R4570 ~ 0.4 with 90% of objects in the range 0.1 to 1 ([322]; [36]). Moderately strong Fe II emission (R4570 > 1) occurs in perhaps 5% of objects, but superstrong Fe II emission (R4570 > 2) is roughly an order of magnitude rarer ([255]). The known superstrong Fe II emitters ([267]; [266]) are listed in table 2; most are luminous/ultraluminous IR AGNs ([267]).

The Fe II lines have the same widths as the broad H lines, suggesting that they arise in the same region ([55]). Fe II is the single largest contributor to the emission line spectrum ([477]). The Fe II emission in most AGNs is probably too strong to be explained by photoionization; the Fe II lines could be collisionally excited in low temperature (6 000 < T < 8 000 K), high density (Ne > 10¹¹ cm^-3) clouds ([210]; [90]; [238]; [444]).

Table 2. Known superstrong Fe II emitters

Name	Position	R4570	Hbr FWHM
			km s-1
PHL 1092	0137+06	6.2	1300
IRAS 07598+6508	0759+65	2.6	3200
Mark 231	1254+57	2.1	3000
Mark 507	1748+68	2.9	965
IRAS 18508-7815	1850-78	2.4	3100

Fe II is strong in objects with weak [O III] emission and vice versa ([55]; [288]). The dominant source of variation in the observed properties of broad line AGNs is a physical parameter which balances Fe II excitation against the illumination of the narrow line region; the anti-correlation could be due to an increase of the covering factor of the BLR as one moves from the strong [O III], weak Fe II objects to the weak [O III], strong Fe II objects; this sort of behaviour is thought to be dependent on the ratio of the actual accretion rate to the ``Eddington accretion rate'' ([55]); indeed, when M >> M_Edd, the disk becomes geometrically thick as radiation pressure becomes competitive with gravity ([30]).

While, for Seyfert 1s and radio quiet QSOs, the Fe II EW distribution extends from ~ 10 to 120Å, the steep spectrum radio sources almost all lie below 20Å. BLRGs and steep spectrum QSOs have weaker optical Fe II and stronger [O III] than either flat spectrum or radio quiet objects ([167]; [211]; [55]).

There is a strong anticorrelation between R4570 and the FWHM of the broad H component ([491]; [463]; [256]). In fact, the Fe II strength and line width seem to be most closely connected with continuum shape in general; strong Fe II emitters have steeper optical spectra, are more X-ray quiet, have steeper X-ray spectra and weaker blue bumps, weaker [O III], and absorption features from outflowing ionized material ([256]).

Six out of 18 QSOs with weak [O III] and strong Fe II emissions were found to exhibit a C IV BAL (broad absorption line) region which is significantly larger than the overall fraction of QSOs observed to have BAL; this suggests a covering factor of 0.33 for the BAL region ([428]).

The relatively rare BALQSOs which show strong Mg II absorption show especially strong Fe II emission and weak [O III] lines; they are also much redder in the interval ~ 1 550-2 200 Å  than the other QSOs ([472]).

All AGNs have strong UV Fe II lines (the Fe II bump at 2 500 Å), regardless of the strength of the optical Fe II lines; the intensity ratio of UV to optical Fe II emission ranges from 4 to 12 (Wills et al. 1985; [94]). The main factor determining the relative strength in the two wavelength bands are the Fe II line optical depths and the Balmer opacity; when the optical thickness increases, the intensity of the optical lines relative to the UV lines increases; a ratio of the order of 1 in the relative intensities of the optical to UV lines implies a column density of about 10²³ to 10²⁴ cm^-2 ([209]; [477]).